An excavator is an example of a construction machine that uses multiple hydraulic actuators or cylinders to accomplish a variety of tasks, such as operation of a boom, an arm, a bucket, and swing. These actuators are fluidly connected to a pump that provides pressurized fluid to extend and retract the actuators for effecting movement of a work tool or tools (implements). Once the hydraulic energy is utilized, pressurized fluid flows from the actuator to a valve. The return fluid typically is at a higher pressure than the pressure in the reservoir and hence contains energy that is wasted once it crosses the valve and enters the reservoir.
To recover energy from the return fluid, the hydraulic system could utilize a servo system in which a regenerative capable electric motor would power or be powered by a fixed displacement, bi-directional pump. A regenerative capable inverter would supply power to the electric motor when the pump is required to provide power to the actuator and would consume and regenerate power to power storage from fluid being returned to either the opposing side of the actuator or to the reservoir.
In a typical unbalanced (differential) hydraulic cylinder, the cross-sectional area of a head-end or extend chamber of the cylinder is greater than the cross-sectional area of a rod-end or retract chamber. When the cylinder is extended, more fluid is needed to fill the head-end chamber of the cylinder than is being discharged from the rod-end chamber. Conversely, less fluid is needed to fill the rod-end chamber than is being discharged from the head-end chamber when the cylinder is being retracted.
A boost system may use a boost pump for supplying fluid to a fluid make-up/communication line that is in communication with inlet/outlet ports of bi-directional pump(s) that supply fluid to the cylinder, and a motor for driving the pump. The make-up/communication line selectively is in fluid communication with one of the inlet/outlet ports of the bi-directional pump when the other of the inlet/output ports is supplying pressurized fluid to the cylinder, thereby to provide hydraulic fluid at a desired inlet pressure to prevent cavitation. A steering system may also be provided with a pump for supplying fluid pressure and flow to a hydraulic circuit separate from the main task actuators.
An auxiliary system may also be provided. The auxiliary system uses hydraulic pressure and flow to turn motors or feed cylinders which operate auxiliary implements, such as brushes, snow blowers, forks, etc. On typical machines, flow and pressure for the auxiliary system may be supplied from the main hydraulic system pumps used to power the steering and/or main implement functions. However, this may cause significant loss in performance to one or more of these functions, since an auxiliary system running at high demand may siphon off too much power from the steering and/or main implement functions.